The present invention relates to compositions and methods related to plugging near-wellbore voids for wellbore strengthening.
Lost circulation is one of the larger contributors to drilling non-productive time. Lost circulation arises from drilling fluid leaking into the formation via undesired flow paths, e.g., permeable sections, natural fractures, and induced fractures. Lost circulation treatments or pills may be used to remediate the wellbore by plugging the fractures before drilling can resume.
Drilling is performed with an overbalance pressure such that the wellbore pressure is maintained within the mud weight window, i.e., the area between the pore pressure and the fracture pressure, see FIG. 1. The term “overbalance pressure,” as used herein, refers to the amount of pressure in the wellbore that exceeds the pore pressure. The term “pore pressure,” as used herein, refers to the pressure of fluids in the formation. Overbalance pressure is needed to prevent reservoir fluids from entering the wellbore. The term “fracture pressure,” as used herein, refers to the pressure threshold where pressures exerted in excess of the threshold from the wellbore onto the formation that will cause one or more fractures in the subterranean formation. Wider mud weight windows allow for drilling with a reduced risk of lost circulation.
In traditional subterranean formations, the mud weight window may be wide, FIG. 1. However, in formations having problematic zones, e.g., depleted zones, high-permeability zones, highly tectonic areas with high in-situ stresses, or pressurized shale zones below salt layers, the mud weight window may be narrower and more variable, FIG. 2. When the overbalance pressure exceeds the fracture pressure, a fracture will be induced and lost circulation may occur. One proactive method to reducing the risk of lost circulation is to strengthen or stabilize the wellbore through the use of particulate wellbore strengthening materials. Wellbore strengthening involves inducing fractures while simultaneously plugging the fractures. This simultaneous fracture-plug method increases the compressive tangential stress in the near-wellbore region of the subterranean formation which translates to an increase in the fracture pressure thereby widening the mud weight window, FIG. 3. The extent of wellbore strengthening, i.e., expansion of the mud weight window, is dependent on the overbalance pressure the plug can withstand before failing, i.e., the plug-break pressure. If the plug fails, lost circulation and drilling non-productive time results.